So if I understand this correctly, you have addresses A.B.C.1 through A.B.C.5
(for example). The router's internal address is A.B.C.1 and the firewall is A.B.C.2
Is your router's DSL interface one of the 5 IPs, or does it have a WAN ip address?
Here's my suggestion - set up a private address scheme, say 192.168.1.x on the inside network of your firewall. Set your firewall's internal address to 192.168.1.1/24.
Create a point-to-point network between your router and your firewall. Set your firewall's external address as 10.1.1.1/30 and your router's internal address to 10.1.1.2 /30
Set a route in the router 192.168.1.0/24 to 10.1.1.1. Then use static translations in the router to map your external IP addresses to 192.168.1.0 addresses. Let's say an internal host is your SMTP server. Set it to be 192.168.1.3. Then use the following
ip nat inside source static 192.168.1.3 A.B.C.3
ip nat inside source static 192.168.1.4 A.B.C.4 (if you have another machine)
and turn NAT on at the router
set dsl interface to be "ip nat outside" and ethernet interface to be "ip nat inside"
access-list 7 permit 192.168.1.0 0.0.0.255
ip nat inside source list 7 interface ATM0 overload
Then make sure your firewall allows the protocol connectivity to the inside hosts.
I think the root of your problem is that you are incorrectly distributing your 5 static ips on different logical networks.
